Flaw detection apparatus using two detectors to assure unbalance in a comparison circuit



1964 s. BERNSTEIN Nov. 24, 3,158,744

' FLAW DETECTION APPARATUS USING TWO DETECTORS TO ASSURE UNBALANCE; IN ACOMPARISON CIRCUIT Filed April 21, 1961 4 Sheets-Sheet 1 ndE mEE.

LEE 8MP 522410 02;.

STANLEY BE RNSTEIN AZTORNEY BCY N QE

S. BERNSTEIN FLAW DETECTION APPARATUS U Nov. 24, 1964 3,158,744 SING TWODETECTORS TO ASSURE UNBALANCE IN A COMPARISON CIRCUIT Filed April 21,1961 4 Sheets-Sheet 2 INVEN'T OR.

STANLEY BERNSTEIN QM umahmwnz m0 zorromecmm nu wmnhzmnz m0 zorromaOmmATTORNEY Nov. 24, 1964 BERNSTEIN ASSURE UNBALANCE IN A COMPARISONCIRCUIT Filed April 21, 1961 4 Sheets-Sheet 3 ELEVATING 74 DRIVE MOTOR J75 /69 TURNTABLE DRlVE MOTOR EYRfiO l ICONTROL 94 I 99L TRANSFORMER wFvfin-To PHASE l I so 5 IZETDECTOR I 91 I \'gl-lggLfi VARIABLE AMPLIFIER95 I sovsnuon RATIO r l CONTROLLED GEAR BOX AMPLIDYNE MOTOR GENERATOR Ii TACHOMETER 98 T0 TURNTABLE 62 I MECHANISM 6| 1 /u 0.0. MOTOR F\G.8 1iI00 1. l I T l I y INVENTOR. I STANLEY BERNSTEIN ATTORNEY Nov. 24, 1964s. BERNSTEIN 3,

FLAW DETECTIQN APPARATUS USING TWO DETECTORS TO ASSURE UNBALANCE IN ACOMPARISON CIRCUIT Filed April 21, 1961 4 Sheets-Sheet 4 FIG. 9

IN VEN TOR.

STANLEY BERNSTEIN A TORNEY United States Patent 3,158,744 FLAW DETECTIONAPPARATUS USING TWQ DE- TECTORS T0 ASSURE UNBALANCE IN A COM- PARISONCIRCUIT Stanley Bernstein, Milwaukee, Wis., assignor to General ElectricCompany, a corporation of New York Filed Apr. 21, 1961, Ser. No. 104,6495 Claims. (Cl. 25083.6)

This invention relates to penetrating ray apparatus and method of usingsuch apparatus for detecting both large and small physical flaws inobjects of interest,'and particularly for unambiguously detecting thoseflaws with data derived from the apparatus and by the method.

It is known in the art to utilize t'wo relatively closely spaced X-raydetectors (or gamma ray, or infra-red or optical frequency raydetectors) for the purpose of ascertaining whether an object in the formof a sheet of metal or the like has disposed therein flaws in the formof slits, cracks, holes, internal or surface pits, or other undesirablestructural characteristics such as non-uniform density or thickness. Thetwo detectors, which receive rays from a source after the rays passthrough the object under inspection, have their outputs combined in acomparison or difference network. The detectors and output network arebalanced relative to each other such that there is ordinarily a nullindication from the diifer ence network. However, when a flaw in theobject passes in front of one of the detectors, an imbalance results inthe amount of penetrating ray energy which reaches the two detectors,with the concomitant result of an imbalance in the output of the twodetectors. In such an arrangement, this imbalance, and therefore thevariation from the null reading that ordinarily obtains, indicates thatsome kind of flaw exists. In short, when one of the detectors viewssomething ditferent from that simultaneously being viewed by the otherdetector, an output signal is generated in the comparison network. Thesign or polarity of the signal indicates which of the two detectors hasviewed the change.

If the flaw is long or large enough, however, both of the detectors mayview the flaw simultaneously. Under such circumstances, each detectorwill have an output signal which may be the same as that of the otherand the net output from the comparison circuit remains a nullindication. Thus, if the flaw is sufiiciently large and of appropriateshape relative to the arrangement of the detectors, it may be viewedsimultaneously by both detectors, and the flaw may pass the apparatuswithout 7 any indication that a flaw exists. It the two detectors arespaced widely apart so as to insure that only inordinately large flawscan be simultaneously viewed by the detectors, a disadvantage developsin that the large spacing between the two detectors will permit manyrelatively small flaws to go by undetected.

In the inspection system described, the motion of the test object inpassing before the detectors, i.e., the scanning pattern and the shapeof the test object, is relatively simple. Thus, if an object such as asheet of metal or other object with a uniform, relatively non-varyingtransverse cross-section is passed with translational motion in only onedirection in front of the two detectors, reasonably satisfactory resultscan be achieved, since the lead edge of the large flaw will almostinvariably cover one of the detectors before the flaw can subsequentlysimultaneously cover both of the detectors. Once this happens, at leastsome kind of imbalance in the circuit develops, indicating at least somekind of a flaw, despite the subsequent null indication that appears Whenthe flaw is viewed simultaneously by both detectors.

In the detection of flaws in either complicated or large objects, theinspection scanning pattern used must of necessity be more complicatedthan one resulting from a simple uni-directional translational motion ofthe test object. Otherwise, the full volume or area of the test objectmay not be completely scanned. With a more complicated scan pattern,flaws (a class of them) exist which, because they are long relative tothe inter-detector spacing and are positioned or shaped appropriatelyrelative to the detectors and scan pattern, cause the undesirablesituation wherein one detector cannot view the flaw at any time withoutthe other detector simultaneously viewing it. With respect to this classof flaws, the coinparison network would steadfastly maintain its nullindication throughout the entire process, and the flaw would goundetected.

Some help for this problem may be obtained by exam ining the output ofsolely one of the detectors (either one) at the same time as the outputof the comparison network is examined. Even though the comparisonnetwork will maintain its null indication, the single detector outputshould show some change in output signal level as the flaw comes intoits view. In a practical situation, however, the flaw may be, and oftenwill be, a relatively shallow and narrow crack, so that the change inthe signal level output from the single detector may be too slight forthe viewer to have any confidence that it is not merely due toexternally or internally generated noise in the system. p

Thus, in the prior art detection systems there is a class of flawswhich, because of its geometrical relationship to the arrangement of thedetectors and the scanning pattern, can, and does, go undetected. Theseflaws have in common, at least, the qualities of being relatively long(i.e., relative to the distance between detectors), and quite shallow(i.e., in the direction measured from the ray source to the detectors).

There are certain applications of flaw detection systems where theundetected passage of long, shallow flaws may not be of much moment.There are other applications for flaw detection systems wherein theundetected passage of such flaws would have catastrophic results.Consider, for example, a flaw detection system utilized for inspectingthe solid fuel motor portion of intercontinental ballistic missiles andrelated types of solid fuel missiles such as the Minuteman and Polarismissiles. The motor section of such missiles comprises a rightcylindrical section some twenty feet in length and five feet indiameter. If a flaw detection system for inspecting such an object whereto permit a substantial number of long, although shallow or narrow,internal slits or fissures to go undetected, the result would be theloss of hundreds of thousands, if not millions, of dollars for eachmissile that would have to be destroyed in flight because of improperfunctioning of the solid fuel motor and such improper functioning maywell result from the failure to detect such a flaw in the motorsection). The dangers to human life resulting from such faulty firings,and thepossible failure to achieve the expected military advantagecannot even be measured in tangible terms, nor

could'our loss of international prestige, nor theneutralization--of"theaggression deterring value of ourmissiles, whichwould surely result from a high incidence of faulty or unsuccessfulmissile launchings.

It is the primary object of this invention, therefore, to provide apenetrating ray flaw detection system capable of detecting substantiallyall classes of physical flaws in an object under inspection, withinacceptable probability with the principles'of the invention by means andmethod for controlling the motion described by, and the distancetraveled by, the test object in'a special way relative-to theinter-detector spacing. These means and method insure that no flawcan goundetected merely because the flaw intercepts, the field of view' ofboth detectors :simultaneously, since they guarantee that at some timethat same flaw will intercept the field'of view of solely one of'the'detectors'andythereby cause an imbalance in the comparison circuit.

T Considering one embodiment, to bedescribed in greater detail below,wherein a solidfuel rocketmotor section isthe test object underconsideration, the motor section network should develop when the leadingedge of the large-flaw covers the first detector and should disappearwhen the flaw covers both detectors.

From a practical point ofview, the invention renders unnecessary themonitoring of solely one detector. Since the ditferential output itselfwill never permit a large flaw to pass without some indication that aflaw exists, in accordance with the principles of the invention, it maybe seen that monitoring the single detector provides little informationthat is not redundant. In the normal procedure for inspection of solidfuel missile motors, once the flaw detection system indicates any signalother than a null, an X-ray picture or radiograph is taken to ascertainthe precise nature of the flaw. The decision is then made as to whetheror not the flaw is of an acceptable nature.

It is a feature of the invention, therefore, that a test object is movedrelative to two penetrating ray detectors 7 to provide a component ofmotion parallel to an imaginary line connecting the two detectors(two-detector line), and a cyclic component of motion in a planeperpendicul-ar to the two-detector line, such that the parallel maybemoved both translationally and rotationally" across, or relative to,the fields of view of the two detectors. Although many inspection orscanning patternsmay be devised 'by dilferent combinations of suchtranslational and rotational motion relative to the, spacing of thedetectors',,one particularly useful scanning pattern involves atranslational component of motion parallel to a lineconnecting themidpoints of the two detectors (twm detector. line) and arotational'component of motion in apIaneperpendicularto the two-detectorline. In such a: pattern, ,a .point projected from one of the detectorsontothe, surface, of-.-tl1e right cylindrical object describes a helixthereon as a result of the combined rotational and translational motionof the :cylinder relative to the detectors. V I

In accordancewith theprinciples of the invention, the distancefiXbetween the two most closely adjacent edges of thewfieldsof view-of thedetectors projected onto the 7 surface. of,.the V cylindrical testobject (inter-detector :dis-

tance), and the length'a of the field of view of one of the detectors inthe direction parallel to, the inter-detector distance, determine themaximum. permissible magnitude forthe pitch of .the scanning patternhelix described on the cylinder. surface, i.e.,t the pitch Y of thehelix must be less than 'or equalrtotwice' (X-I-a). Thus, for everycomplete rotation of the cylinder, the cylinder travels a 1 distance Yparallel to its longitudinal axis (and therefore parallel to thetwo-detectorline) which is'less than or equal to. the distance 2(X-1-a).This insures, as will beseen, that no flaw'anywhere on or within thecylinder 'ential' output from both the detectors, to'help. insure thata;large,.shallow flawwill not go undetected. For example, suppose thatonrthe output from" the Isingledetector,, there appears a variation inthe'signal 'level which seems tube, but is not clearly, morevthanthat'which could be attributedto noise. Immediately, then, the output of"the comparison .ori ditferential network is examined to V see if itverifies the existence of a flaw. If there is, in fact, ya largeshgallow fiaw nn imbalance :in the comparison,

component of motion has a magnitud'eless than or equal to twice the sumof the distance between the most closely adjacent edges of the fields ofview of the two detectors during any one cycle. defined by the cycliccomponent of motion, and the length of the field of View in thedirec;tion of the twodetector line. g

It is another feature of theinvention thatin the inspection of acylindrical object, the cylindefflis moved translationally androtationally relative to two detectors 'suchthata helix is described onthe surface of v the cylinder by a point projected on to the surface .ofthe cylinder from one oftthe. detectors, and that meansis provided to,insure that the pitch of the helix is lessv than or equal to twice thesum-of distance betweenthe two most closelytadjacent edges of thefieldsof viewiformed on the cylinder by the'two detectors, and the length ofthe field of viewin the same direction. f

' The detection of flaws in certain large objects such as; for example,the cylindrical solid fuel motor section 20f the missiles mentionedabove, is further. complicated by cylinder, or includes any other line.within' the cylinder i pa'rallelto the axis, and also includes the twodetectors f on one side ofthe test, object .and the penetrating ray;

complex internal void' geometries that arecharacteristic of the internalconfiguration of the 'objectand normally occur therein.- For example, inthe motor section of the missile, a largesixepointed star aperture,-orhollow, passes a through the entire longitudinal extent; ofthe cylinder,

such that at any transverse cross-section along thecylin der, anaperture in the form of a six-pointed starr'nay be seen. It isobvious'that detecting a small flaw in the'fform of a thin 'slitor pitin' such an object, which normally and desirably has large internalvariationsin its cross-'sectionalpshape, tis'exceedingly difficult; a a

It is an additional object of this'invention, therefore,- to provide apenetrating ray flaw detection system which is capable-of detectingrelatively small flaws in. the press ence of regular, expected, largeandkcomplicated internal physical variations in theobjectunderinspection;

The above object isjaccomplished in; accordance with the principlesofthe invention by arranging-theutwo detectors relative tojthe objectunder examination such that each; detector, at every point in'time,views preciselyt-the same transverse cross-sectional configuration ofthe. object "as does the other detector. Byinsun'ng thisiirelationship,

the comparison network" is always balanced 'to provide a nullindication, except whenta flaw appears under one;

but not both,- of the detectors.

This is done by aligninglthe two detectors along-a line of zero orminimum change in cross-sectional shape; Q or more specifically, alongthe direction of zero or minimum gradient in, cross-sectional .change,Consider an imaginary plane passing through the cylindrical star-holedtest object which includes the; longitudinal axis of the source on theother side of the test object. Such a plane cuts through the diameter ofa transverse circular crosssection of the cylinder if it includes thelongitudinal axis of the cylinder, but will cut through such across-section as a chord if it includes a line parallel to, but otherthan, the axis. In any event, the two detectors are spaced relative toeach other in the plane to define a line (twodetector line) which isalso parallel to the longitudinal axis. The result of this arrangementis that parallel lines projected from the two detectors throughrespective cross-sections of the test object and in the imaginary planedescribed, will intercept, in their passage through the test object,precisely the same cross-sectional configurations. This remains true forwhatever motion the cylindrical test object experiences as long as it iseither translational motion or rotational motion about its longitudinalaxis.

In contrast, should the detectors be ali ned in any other plane, i.e.,any plane at an angle other than parallel to the longitudinal axis ofthe cylinder, the two detectors, under most portions of the scanpattern, would simultaneously view different cross-sectionalconfigurations, resulting in an imbalance and therefore an output fromthe comparison network. In these circumstances, the output from thecomparison network during the scanning pattern is a complicated pattern(rather than a null) from which the detection and recognition of a flawindication is extremely difficult, if not impossible under mostcircumstances.

In accordance with the invention, however, the two detectors aligned inthe direction of zero or minimum gradient in test object cross-sectionalchange, view the same physical configuration at all times, except when aflaw passes in front of one of them.

The novel features which I believe to be characteristic of my apparatusand method invention are set forth with particularity in the appendedclaims. My invention itself, however, both as to its organization andoperation, together with further objects and advantages thereof, maybest be understood by reference to the following description taken inconnection with the accompanying drawings.

In the drawings:

FIGURE 1 shows a preferred embodiment of the flaw detection system, inaccordance with the principles of the invention, wherein a cylindricaltest object is under inspection;

FIGURE 2 is a graph showing curves descriptive of the power outputcharacteristics of the penetrating ray source used in FIGURE 1;

FIGURE 3 is a graph showing curves representative of the output voltagesfrom each of the two balanced detectors of FIGURE 1;

FIGURES 4A, B, C, D and E represent temporally successive positions andconditions of inspection of the cylindrical test object of FIGURE 1subject to the scan pattern in accordance with the principles of theinvention;

FIGURE 5A is a graph of the idealized output signal from the comparisonnetwork of FIGURE 1;

FIGURE 53 is a graph of the idealized output signal from solely one ofthe two balanced detectors of FIG- URE 1;

FIGURES 6A and 6B are the same as FIGURES 5A and 5B respectively, exceptnoise has been impressed on the curves;

FIGURES 7 and 8 show an example of an embodiment which may be used tomove the cylindrical test object of FIGURE 1 on the one hand, and thedetectors and penetrating ray source of FIGURE 1 on the other'hand,relative to each other to provide the scan pattern of FIGURES 4A-E; and

FIGURE 9 is another embodiment of flaw detection system in accordancewith the principles of the invention.

Referring with greater detail to FIGURE 1, there is disclosed apreferred embodiment, shown for purposes of illustration, of a flawdetection system utilizing apparatus and the method in accordance withthe invention. In this embodiment the object subject to flaw detectionis one which is very large and has a complicated internal cross-section.It requires, therefore, a source of hard penetrating rays and detectorscapable of sensing the hard rays that emerge from the penetrated testobject. X-rays are the penetrating rays utilized in the embodiment ofFIGURE 1 for the flaw detection function.

Considering FIGURE 1, then, there is disclosed a source of hardX-radiation, the betatron 11. Betatron 11 and its output window 12 arearranged such that a 15 degree conical beam 14 of X-radiation passes tothe right. Immediately in front of output window 12 of betatron 11, isdisposed an ionization chamber 13 such that substantially all of theoutput from window 12 passes through ionization chamber 13 and continuesto the right. However, a small amount of X-radiation is monitored ordetected by chamber 13 through the ionization caused within the chamberby the radiation. Ionization chamber 13 merely sarnp'es a minute portionof the output penetrating ray energy from betatron 11, and because ofits location relative to the aperture 12, results in no shadow of theionization chamber being cast by the penetrating ray energy in itsprogress to the right in FIGURE 1.

Disposed in the path of X-ray beam 14, is a test object 15 which isright cylindrical in shape, approximately five feet in diameter andtwenty feet in length. Characteristically, this object may be the motorportion of a solid fuel ballistic missile. The disposition of rocketmotor 15 is such that its longitudinal axis is vertical. Rocket motor 15has disposed therein a six-pointed star aperture 16 which penetrates theentire vertical length 7 of the internal section of cylinder 15. Staraperture 16 is a full length bore through the cylinder 15 and is coaxialtherewith. Typically, the rocket motor portion 15 comprises a materialwhich is of a rubbery or cheese-like consistency.

To penetrate so large and dense a test object and obtain usefulinformation, the X-rays emanating from source 11 must have a high energylevel. For this reason ordinary X-ray tubes are not satisfactory, andthe higher output level betatron 11 is required. Typically, the outputof betatron 11, which may be measured prior to its passage through testobject 15 is capable of a 25 million electron volt level. A distributioncurve 21 of the energy output of betatron 11 is shown in FIGURE 2,wherein the abscissa is the energy level in millions of electron voltsand the ordinate represents the quantity of radiation or photon flux.Curve 22 represents the effective radiation which remains after beam 14passes out of test object 15. These curves show that the lower energylevel X-rays are absorbed by test object 15 and only very h gh energylevel radiation is available at the output or right-hand side of thecylinder 15. Consequently, the detectors that receive X-radiationpassing out from the test object 15 must be capable of operating at veryhigh energy levels.

These detectors are distinguished from the ionization chamber 13immediately at the output of betatron 11, in that relative to detector13 it is necessary to have substantially all the radiation passtherethrough Without isturbing the general pattern and distribution ofX-ray beam 14. The function of the ionization chamber 13 in the overallsystem will be discussed after a general description of the entirearrangement of FIGURE 1 is presented. Consequently, for the purposes ofthe immediate discussion, the purpose and operation of ionizationchamber 13 should be ignored.

Disposed to the right of rocket motor 15 and on the opposite sidethereof from betatron 11, is a housing 17 which includes ionizationchambers 18 and 19, with which the X-radiation 14, emerging from testobject 15, communicates through two apertures 23 and 24, respectively.Housing 17 is preferably metal having a heavy atomic reaching.theionization -chambers 18 and 19'do so through thehaperturesr-zland24,'respectively. Considering. the general geometric relationship of theapertures 23-24, the, ionization chambers 1849, and the housing 17, itmay be seen that the housing 17 is essentially a solid rightparallelopiped with the apertures 23 and '24, in anytransversecross-section, having a rectangular shape. Apertures 23-24 aredisposed vertically, one above each other, and are. preferablydimensioned one inch vertically and one-half inch horizontally, with themost adjacent edges, '25 of aperture 23, and 260i aperture24, spacedfrom each other two inches. Apertures 23 and 24 extend-to the right intoho'usin 17, a. distance of approximately eight inches.

The function of apertures 23. and 24 is to collimate the X-rays enteringthem. after their exiting from the test object 15. The X rays thus passas parallel rays through the apertures and exit in that form at theright hand ends of the apertures. The length of the apertures and thedistance from theX-ray source, 11 results in the apertures properlyfunctioning as .collimatorsv To insure that the.

collimating function isproperly performed, however, it may bedesirable.to include in the apertures Xeray collimating slits,.sometimes referredto as Soller slits; These slits, which are well known in the art,comprise an array of thin lead sheets, orrsheets of heavy atomicweight,"

which are spaced from each other and arrayed with their faces parallelto each other and to the direction in which the collimation is to beperformed.

I The collimation provided by apertures 23 and 24 re.- sults in parallelbeam X-rays emerging at theright-hand ends of the aperturesgandimpinging on and passing into the ionization chambers 18' and 19respectively. In a. similar way, the'function of. the apertures 23 and24 may be ,viewed in the" reverse conceptually. Thus, a field of view,inithe form of a small rectangular area, may be pro jected on to (andthrough) test object 15 from each of the apertures 23 and 24. Inessence, then, the apertures 23 and 24 define a field of view on thesurface of, and through the voluine of, cylinder 15 in a manner suchthat the ionization chambers 18 and 19 can be visualized as looking on tand through thetest object 15. with thefield defined by the shapeofapertures 23 and 24.,

The ionization chambers 13 and 19 are right cylinders. They aresubjected to high X-ray energy levels, represented by curve 22 ofPIGUREZ, havingan effective radiation of approximately l0. millionelectron volts. They are,

therefore, designed so as to sense a sufiicient portion of this highenergy X-radiation. Thus, the ionization chambers 18 and 19 aretypically filled with xenon, which is one'of therheavy atomic weightinert gases, at a pressure.

often atmospheres. The size, shape and pressure within a the twoionization chambers are made as closely similar to eachother aspossible, so thatthe ionization chambers will ,be' as closelybalanced'detectors as is possible. In all respects, except for the factthat they are designed to opcrate. with such high energy level X-rays,the ionization chambers are typical ionization chambers known toth oseskilledinsthe art. These ionizationchambers are cyline drical in shape,so as to sustain theten atmosphere pressure of thenenclosed'xenon;Chambers of this type are available from theGeneral Electric Company,X-Ray Bepartmenn'lviilwaukee, Wisconsin.

' The electrical outputsof detectors i and 19 are connected to thepreamplifiers 27 and 28-, respectively.

Amplifier 27', however, is adapted to provide ,an output voltage signalof opposite polarity to that of amplifier 28,

in manner well known to those skilled in the art. Preamplifiers 27 andZSare drift-compensated DC. operation- .al amplifiers, well known tothose'skilledin the art The voltage outputsof, amplifiers -7 and 28 areconnected through scalingresistors '29 and 31'}, respectively; and

1 fthence through a balancing potentiometer 31-t0 a-summing amplifier32.. Amplifier 3211215 a feedback resistor which, in combination,performs the typical D.C.' analog tion chamber outputs are supposed toindicate a balanced.

condition or equal magnitude outputs. The output of the summingamplifier 32 isthen fed, via conductor 34, to a typical two-channelrecordingdevice 35. i

Test cylinder 15'is adapted to be moved,.by"means to be .discussedingreater detail. relative to FIGURES 7 and 8, in front of and past thevertically aligned apertures 23 24 in amanner now to be brieflydescribed. The cylinder 15 is mounted on a horizontal jturntable(notshownin- FIGURE 1) in a manner such that it is rotated about itslongitudinal axis. Additionally, the cylinder. 15 moves verticallyupwardly relative to apertures 'ZBand 24. Apertures 23 'and 24 arealigned'in a vertical plane which passes through the longitudinal axisof object 15 and through 'betatron 11 at its output aperture 12; Inthis'way, the-fields of view of cylinders-184?, through apertures 23:and 24, are in theform of two small rectangles, one-above the other,which'penetrate diametrally through the rocket motor 15. I Thevertically upward translational motion afforded the cylindrical testobject 15 and the simultaneous rotational.

motion about object 15s longitudinal or verticalaxis results in ahelical scan'of the rocket motor 15 by the apertures 23 and 24; Moreparticularly, iftheaxis of -aper-- ture 23 is extended to the left to'apoint on the surface of cylinder 15, thecombined relativetranslationaland rotational motion of the cylinder would-resuit'in that point,alfixed to the axis of aperture '23, describing a helix on the surfaceof the cylinderi V V The internal geometry of cylinder 15 is such thatvaria; tions in the internal configuration do not occur in averticaldirection, but only in a horizontal sense. Since the twodetectors-1819,' and their cooperating apertures 2324,* are verticallyaligned, they are on a line of zero gradient in cross-sectional change,i.e., from a point at thetop of the object'15 to a point'at the bottomof the object-15 in' a direction parallelto the longitudinal axisthereof, there is no variation to be observed in-the transversecross-seek tion as you move from one point to the other. Variationscouldonlybe observed alonga line-relative to'the cylinder that hasanon-vertical,comp onent of direction. Therefore, the apertures 23-24;literally-viewthe same type of internal configuration at alltimesxduring the entire helical scanning pattern: 'Although in the"-embodiment of FIG- URE 1 the apertures are aligned infa vertical planewhich intersects the longitudinal axis'ofob-ject15 and the aperture 12of betatron 31, they mayalso, if other'circum stances indicate thedesirability, be located on a-plane which passes through theobjectto'the right or to the left of the longitudinal centerline of object-15;In thisar:

rangement, even though the'plane'is off-centeriland. formsa chordal,rather than-diametral plane relativeto' ationprojected from betatron ll,the functioning of the t flaw detection systemrernains essentially thesame as that 7 described above, 7 a V a Let us consider the operation ofthe embodiment of the FIGURE 1 with the test object 3.5 in a 'stationarycondi ti'on. f Eetatron 11 projects a beam of Xrays 14, out' through itsaperture 12 .(andwhich passes through .detec tort13 relativelyunhampered and undisturbed) which has a photonfiux versus energydistribution as represented by curveZLof FiVGURE Z, I Itpassesthroughthe motor and in the process, the photon flux versus energy distributionof the X-ray beam changes as represented by the curve 22 of FIGURE 2A,which is the condition of the beam on emerging from the right-hand sideof cylinder 15. The beam then enters the apertures 23 and 24 which,because of their identical size and geometry and because of theirviewing precisely the same configurations of object 15, permit theentrance of substantially equal amounts of photon flux which in turn issampled by the ionization chambers 13 and 19, respectively. The outputsof the ionization chambers 13 and 19 are amplified by preamplifiers 27and 28, respectively. Because of the phase inversion provided byamplifier 27, the output voltages from the two amplifiers are addedalgebraically through the equal valued scaling resistors 29 and 30 andsumming amplifier 32, to provide a null signal on conductor 34 and anull indication on recorder 35. The function of the potentiometer 31 asan input coupling to the summing amplifier 32 is to trim the inputsignal to insure a precise null output from the summing amplifier incase some imbalance in the ion chambers or apertures has occurred whichis not otherwise subject to elimination. It should be noted that oncepotentiometer 31 is trimmed with wiper 33, the null indication willcontinue to remain a null, even if thereis fluctuation in the photonflux output of the betatron 11, since each of ionization chambers 18 and159 will experience precisely the same variation as the other. Thus, thefluctuation in betatron output is balanced out in the comparison networkcomprising the amplifiers 27- 28, resistors 29, 3t and 31, and summingamplifier 32.

Let us now consider the operation of the embodiment of FIGURE 1 when thecylindrical object 15 is moved in front of the detector to provide thehelical scan pattern described above. Assume the rotation andtranslation commences with the field of View through apertures 23 and 24across the cylinder 15, as is shown in FTGURE 1, such that the viewinitially is in a plane including line 38, shown across the topcross-section of cylinder 15. In such an orientation, each aperturereceives a minimum amount of penetrating ray energy, since line 38passes through the maximum amount of material that can be interposedbetween betatron 11 and the detectors 18 and 19. In one completerotation there is experienced six such maxima in terms of amount ofmatter interposed, or six minima in terms of the amount of penetratingray energy reaching the detectors. These correspond to the six recessedpoints of the six-pointed star. There are also, for each completerotation, six maxima of penetrating ray energy reaching the detectorscorresponding to the six points of the star, since in those orientationsthe minimum amount of material is interposed between betatron 11 and thedetectors 18-19 Reference to FIGURE 3 shows curve 41 representative ofthe output from preamplifier 28 which is coupled from the output ofionization chamber 19, curve 42 shows the output from amplifier 27 whichis, in turn, coupled from the output of ionization chamber 18. Thedistance along the abscissa represents one complete rotation of cylinder15. The polarities of curves 41 and 42 are opposed, due to the phaseinversion of amplifier 27. Since the two detectors are mountedvertically one above each other such that the line joining them is onealong which no change in the internal confi uration of the cylinder isexperienced, at every instant of time, the ionization chambers 13 and 19receive precisely the same amount of penetrating ray energy passingthrough the cylinder 15. Summation of the curves 41 and 42 (which isprecisely what is done with the output of amplifiers 27 and 28 in thesumming amplifier 32) results in curve 43. This is nothing more than azero output or null signal which is effectively the signal recorded onthe two-channel recorder and applied thereto on conductor 34 from theoutput of summing amplifier 32. Thus, when there are no flaws in thetest object the sum of two equal but opposite waveforms provides a nullindication.

If a flaw exists and is viewed by one, but not both, of the apertures23-24, however, the amount of X-radiation that reaches one of the twoionization chambers 18-19 is different from that for the otherionization chamber. This results in a variation in either curve 41 orcurve 42, but not both, which in turn causes an imbalance; accordingly,a pulse or irregularity in curve 43 appears to indicate that a flawexists.

From the curves of FIGURE 3, it may appear to the observer thatinformation as to whether a flaw has passed in front of a detector mayalso be obtained by looking exclusively at one of the two curves 41 or42; let us say, for example, curve 42. This is so because the regularpattern shown for one period corresponding to a complete rotation ofcylinder 15 is quite regular, and would continue to be so with repeatedrotations. One could assume, therefore, that if a flaw appeared in thecylinder, it would appear as a variation in the regularly recurringpattern of curve 42, for example. However, other possibilities forvariation in the regularity of the curve 42 may well occur. For example,the output of betatron 11 may fluctuate in accordance with variations inline voltage and this would result in an irregularity in the pattern ofcurve 42. Curve 42 would therefore seem to indicate a flaw when in factno flaw existed. This is due to the absence of a balancing of the outputfrom amplifier 27 against another signal which includes the same kind ofvariation.

To help detect a flaw by looking at the output of solely one detectorwithout getting a false indication because of variations in betatronoutput, the monitoring detector 13, mentioned before, is utilized. Morespecifically, ionization chamber 13 monitors the output of betatron l1.Detector 13 is in many respects similar to ionization chambers 18 and1?, but it is filled with one atmosphere of krypton rather than tenatmospheres of xenon, so that most of the radiation passes byundisturbed. The output of monitoring detector 13 is then applied topreamplifier 44, which amplifies the output of chamber 13 to a smallerextent than do the amplifiers 27 and 28 relative to ionization chambers18 and 19. Furthermore, the output of preamplifier 44 is applied acrosspotentiometer 45, so that a voltage appropriate to the level needed maybe tapped off as .resired. A potentiometer wiper is connected to ascaling resistor 46 having a resistive value equal to that of scalingresistor 47 in the output network of preamplifier 27. Both scalingresistors 46 and 47 act as inputs to the summing amplifier 43 throughthe balancing potentiometer 49. In this way the output of monitoringionization chamber 33 is balanced against the output of the ionizationchamber 18; the output from the comparison network formed by theamphfiers 44, 27, scaling resistors 45-47 and summing amplifier 48 is asignal much like curve 42 of FIGURE 3, but displaced upwardly since itis of smaller magnitude due to the subtractive signal provided by themonitoring detector 13. The output of summing amplifier 46 is thenapplied along conductor 51 to the two-channel recorder 35. In this waycurve 43 of FTGURE 3 is applied to the recorder 5 on conductor 34, whilecurve 42, compensated, however, so that any variation in betatron outputis neutralized, is applied to the two-channel recorder 35 alongconductor 51.

Let us now consider the operation of the embodiment of FIGURE 1 inaccordance with the principles of the invention, when a long, narrow andshallow flaw appears on the surface of the cylinder 15. Consider theseries of FIGURES 4A through 4E for this purpose, with attentionpresently being directed to FIGURE 4A. Shown therein is a small portionof cylinder 15 with a flaw 40 in the form of a long scratch which may beseveral inches in length, and in any event, longer than the sum of thedistance X between edges 25-26 of apertures 23 and 24, and the verticallength a of aperture 24. Flaw 40 may have a shallow depth, perhaps lessthan an inch, and a width of about .01 of an inch.

Disposed upon the surface of the portion of cylinkites and rotates.

of FIGURE 1, Xris equal to two inches, and a to one inch. The solidlines 47 and 48 are imaginary lines" depictingthe helical scanningpattern on that surface of the cylinder facing the detectors. The brokenline 51 isthat portion of the helical scan pattern represented on theopposite surface of cylinder 15, that is, on the side nearest betatron11'. The distance between lines 47 and .48, designated 'as Y, 'is thepitch of the helix, i.e., the total translational or vertical distancethat the cylinder travels for one complete rotation of the'cylinderabout its longitudinal axis. As required bythe invention, and forreasons which will become'more apparent shortly, the relationship of Y,X and a is that Y is less than or equal to 2(X+a), i.e., V2(X+a).

Consider now the relationship of the flaw 40 w the projections ofapertures 23fand 24 as cylinder '15 trans- The projections of apertures23 and 24 should be considered stationary in space, and the trans lationand rotation of the cylinder 'results in the flaw following the helicalpatternrepresented by lines 47-49 and 51-52. The view presented inFIGURE 4A shows the leading edge of the flaw 40 having reached thelower- "most edge of aperture 24, but not yet within the field of viewof the aperture. Obviously, as the aperture passes along the helix, itwill pass by the lowermost edge of the aperture 24 without interceptingthe field of view of that aperture. 'i V 7 FIGURE 4B shows the situationafter cylinder 15 has been rotated 180. Flaw 40 is represented withbroken lines to indicate that now it is on'the far side of the cylin-'der from the detectors. The leading edge of 'fiaw 40 has arrived "alongthedotted line 41 of the helix to a point just before the lowermost edgeof aperture 23, and therefore doesnot intersect the field of view of:aperture 23.

However, the middle portion of flaw 49 now intercepts the view ofaperture 24. This results, as shown in the curve of FIGURE 5A, inapositive-goingpulse 54 due to the imbalance that results in thecomparison network output from the two detectors. 7,

FIGURE 4C shows the position resulting from an addi; tiona1'180"rotation over that of FIGURE 4B, or one complete rotation from theposition of FIGURE 4A.

Here, flaw intercepts the'fields of view of both detectors23 and 24.Consequently, there is no imbalance in the output comparison network ofthe two detectors (we are assuming'that long, shallow flaw 40 isrelatively unithese figures.

of View, of aperture 24, isintercepted,

channel. FIGURE 5B is essentially the same curve as Now, however, a flawis in- The curves shown in FIGURES 5A and 5B are idealized versions ofwhat would'appear on the chart ofthe outputrecorderSS. If the entiresystem were free of noise, one could expect nice, smooth curves shown inAs a practical matter, the noise'generated in .a system such'as thisresults in indications that are similar to those represented in FIGURES6Aand, 6B. Tnesefigures are merely the curves of FIGURES 5A and 5Bchanged to include a reasonable amount of noise for such a system. flaw,the more difficult will it be to. distinguish pulses 54, 55 and 56,since the chance will be greater that they will be masked by noise.

It maynow be understood how the relationship'of Y being less than'orequal'to 2(X-i-a) prevents'the-"undetected passage of any long, shallowand/ or narrow-fiaws;

Under this constraint, it is not possible for flaw 40 to of 'view ofaperture 24 alone, or 2.3 alone.

close to being one wherein there is actual danger of a flaw goingundetected without its actually happening, since in those figures Y ZQI+11). It may-be noted that the leading edge offlaw-40 in FIGURE 4A justmisses the field of view of aperture 24. After a 180 rotation as shownin FIGURE 4B, the'leachng edge of flaw 40 just misses the field of Viewof aperture 23: while the field Obviously, if the pitch of the scanninghelix is greater than that'disclosed in these figures, the leading 'edgeof flaw 40would' intercept the field of View of aperture 23." in FIGUREIB, and both detectors-would view the flaw-with a concomitant null.Therefore, there would not be satisfied the necessarycondition for theunambiguous detection of large flaws, i.e., no situation should developwherein both detector fields of View are intercepted @without the situation having first developed that solely one field of view formthroughout its length). Accordingly, FIGURE 5A- a shows'a 'nullindication maintained at the point correspending to the FIGURE 4Cposition, i.e., after one complete rotation of the cylinder.

With an additional 180 rotation as represented in FIGURE 4D,'flaw 40 isonce again on the far side of the cylinder. Now, however, its'laggingend intercepts aperture 23, while-the view of aperture 24 is compietelyfree .of-any flaw. Consequently,;an imbalance signal is once moregenerated;; this results'in anegative going pulse in-the curve of FIGURE5A.; The negative polarity is due to the -fact that the output amplifierfor aperture 23 is of opposite polarity from that for aperture 24.

Andlastly,FIGURE 4E shows the cylinder rotated an additional 180, i.e.,-two complete'rotations beyond FIG- URE14A, so that 'iiaw 40;is nowcompletely beyond the 'fields of view of both the apertures, and ofcourse FIG- URE 5A- indicates a null.

. 1The: curve of FIGURE 5B represents'the monitor 13 compensated outputfrom detector 18-apert1ire 23, which is applied from summing amplifier48 to recorder 35 on conductor-51. This input co-recorder 35 willhereinafter he referred to asthe second channel while-the curve ofFIGURE 5A, representing the signal applied to re corderfiscunductorEq,will be refeired toas the first is intercepted. In short, ifthe pitch Y were greater. than the pitch shown in FIGURES 4A" and 4B,the very firstv interception of either of the two fields of view wouldhave occurred simultaneously with the interception of the other of thetwo fieldsofview.

Where the dimensions of the apertures are relatively small compared tothe distance between the apertures,

i.e., where d is very muchsmal-ler than X, then the re- 7 quiredexpression simplifies to Y ZX. i a a In the operation of the flawdetection system of. FIG- URE 1, the indication on the output recorder35 of any kind of'flaw at all is a signal to the operator to stop theprocess completely and to take a radiographic picture of that portion ofthe test object to ascertainv precisely the size, shape and natureof'theflaw previously detected. Some types of flaws may 'be acceptable, Whileothers may not. Where such a radiognaphioprocedure is followed,- thesecondchannelof FIGURE 1 and all its related equipment and circuitry maybe dispensed :with. It-may be recalled that reference to a secondchannel (i.e., the out;

put of oneof two balanced detectors) may have proved .7 helpful in apriorart two-balanced detector system, since a large flaw may havegoneundetectedin the prior art system if the flaw was of the type that wouldsimultaneously intercept'both detector fields of view without" havingintercepted, at some other unto, the field of view of solely onedetector; Under thosecircumstances, reference to the single detectoroutput might haveindicated 7 Clearly, the shallower or narrower; the.

13 the existence of the large flaw if it had not been too shallow and/or narrow a flaw. If it was too shallow and/ or narrow, the indicationwould not show above the noise level in the single detector output.

Now, however, in accordance with the principles of the invention, it isno longer possible for a flaw to go undetected merely because itsimultaneously intercepts the fields of view of both apertures Withoutat some other time intercepting the field of view of solely one.Accordingly, the ambiguity is removed and reference need not necessarilybe made to a single detector (the second channel of FIGURE 1).Accordingly, the second channel with its related monitoring circuitrymay be eliminated if desired. This may be particularly appropriate whenthe procedure is followed wherein a radiograph is taken of the testobject responsive to a first channel when there has been a flawindication. However, the conservative approach may be indicated undersome circumstances; then both channels should be retained sinceredundant information may be useful, if only for the purpose ofverification The relative motion between cylindrical test object 15 anddetectors 18-19 and betatron 11 needed to satisfy the requirementsdescribed above, may be accomplished in several ways. For example, itmay be desirable to move the object 15 rotationally about itslongitudinal axis while simultaneously moving the detectors and thebetatron translationally relative to the cylinder. Alternatively, thedetectors and betatron may be rotated about the cylinder while hecylinder is moved translationally parallel to its axis; or the cylinderitself may be subjected to both longitudinal and rotational motion, orsimilarly for the detectors and betatron.

In any event, the two components of relative motion result in therequired helical scan pattern, and may be constrained in accordance withthe principles of the invention described above. One system forproducing such motion will now be described. It is one wherein thecylindrical object is rotated about its longitudinal axis upon aturntable while the detectors and betatron are moved translationallyrelative to the object.

Consider now the system of the embodiment represented in FIGURE 7. Thecylindrical rocket motor 15 is mounted with its longitudinal axis in avertical position upon a turntable mechanism 61. This turntable isdriven, through appropriate gearing, by .a D.C. motor 62. The D.C. motoris in turn controlled by circuitry to be described in greater detailhereinafter. Surrounding and disposed above rocket motor 15 is asuperstructure having four vertical legs 6366 at the top of which aredisposed four horizontal beam members connecting the upper ends of legs63-66 to form a rectangular frame. Located below and parallel to thehorizontal frame formed by members 67-70 is a horizontally disposed butvertically movable platform 71 having an opening 72 therein, anddisposed relative to rocket motor 15 such that the vertical motion ofplatform 71 may occur with rocket motor 15 passing freely within opening72.

Mounted and secured to the lower portion of platform 71 is detectorhousing 17 (of FIGURE 1) showing the detector apertures 23 and 24, andalso betatron 11. Betatron 11 and detector housing 17 are secured toplat- .form 71 so as to maintain the relationship to each other and torocket motor 15 as disclosed and described relative to FIGURE 1.

Platform 71 is rendered vertically movable in the manner now to bedescribed. An elevating D.C. drive motor 73 is mounted upon horizontalmember 68 and engages drive shaft 74, which in turn, through gearingdrives related drive shafts 75, 76 and 77, mounted upon horizontalmembers 67, 7t) and 69, respectively. Through appropriate gearing, thedrive shafts drive four vertical lead screws 89, 81, 82 and 83, whichare disposed adjacent to and parallel with Vertical legs 63456,respectively. Four lead screws 80-83 are threaded through the 14 fourcorners of movable platform 71 through threaded apertures 84-87,respectively. Two counterweights 88 and 89 are secured to movableplatform 71 through Wire tackle over blocks appropriately disposed.

In operation then, turntable mechanism 61 is driven at a steady rate ofturntable drive motor 62. In this way, rocket motor 15 rotates in placeat a steady rate of rotation. Simutlaneously, detector housing 17 andbetatron 11 are moved vertically upwardly (or downwardly, as desired)relative to rocket motor 15 by virtue of D.C. elevating drive motor 73actuating drive shafts 74-77, which in turn couple to four lead screws83 driving movable platform 71 (and aided by counterweights 88 89). Therate of vertical movement of the platform, and therefore of the betatronand detectors, is controlled by the speed of elevating drive motor 71.This speed is fixed relative to the gearing and to the rate of rotationof turntable 61 such that the vertical movement of platform 71 during asingle complete rotation of turntable 61 is less than, or equal to,twice the sum of the spacing between apertures 2324 plus the verticallength of one of the apertures.

In order to assure a steady and reliable turning rate, and to be able toconstrain the Vertical and rotational motion in accordance with therequirements of the invention, an eificiently controllable and sensitivedriving circuit is required. Such a circuit is disclosed in FIGURE 8.This figure is a diagram of an appropriate turntable drive system. ShownWithin broken line box 99 is a variable speed, governor controlled motor90 connected to drive variable ratio gear box 91, whose output shaftdrives synchro control transmitter 92. The output leads of the synchrotravel to synchro controlled receiver and transformer 93 which may bemounted closely adjacent to D.C. turntable drive motor 62 (shown both inFIGURE 8 and at the bottom of FIGURE 7). The output of the synchrotransformer is coupled to amplifier and phase detector 94 whose output,in turn, is coupled to amplidyne generator 95 which directly drives D.C.motor 62. Motor 62, in turn, through appropriate gearing, drivesturntable mechanism 61. Geared oif the drive shaft driven by motor 62 isthe synchro transformer 93 whose output is fed back, as part of afeedback loop, to phase detector and amplifier 94. In this Way, thespeed of the D.C. motor may be compared with the rate of variable speedmotor 99 as transmitted to synchro transformer 93 to insure that thereis no rate error, and if a rate error is generated, a correction signalmay be immediately applied to amplidyne generator 95. The tachometer 96is driven off the gearing of the drive shaft of motor 62 and has itsoutput coupled to detector 94 ot insure servo stabilization.

It may be readily understood that the same circuit may be used fordriving elevating drive motor 73. Appropriate gearing 97 may be used totake the mechanical output of variable speed motor 90 and apply it viashaft 98 to another variable ratio gear box to then drive the samecircuitry as shown in FIGURE 8, but with the output applied to drive 73.Shaft 98 is shown coupled to empty broken line box 16% whose contentswould accordingly be the same as that of box 99. By appropriatelyselecting the gear ratio in gear box 91 and the gear ratio to be used inthe elevating drive mechanism, the required relationship of pitch toaperture spacing may be readily obtained, in accordance with theprinciples of the invention.

The principles of the invention are equally applicable to lesscomplicated test structures than rocket motors. FIGURE 9 discloses anembodiment, given for purposes of illustration, in accordance with theprinciples of the invention, wherein test object 161 is a fiat metalsheet which may, for example, be part of a production line or theproduct of a rolling mill. Metal sheet 191 may be on its way from havingbeen rolled into its indicated shape. In order to detect flaws in thissheet of 'metal, X-ray -tube .162 is disposed above test sheet ltll,

such thahXdadiationfrom tube ltil'passes vertically downwardly atrightangles to the plane of object 101. In this embodiment, an ordinary X-raytube is used, ratherthan the betatron of FIGURE 1, since the testobject'is relatively thin and accordingly does not require as-high anenergy level as was required for detectors-18 and 19; Two apertures 105and 106-are disposed in housing 104,-which are the counterparts ofapertures'23 andifdof-FIGURE 1: 7

Test sheet, 101, as part of the production line movement,

' is caused to move translationally between tube 102 and housing 104.Simultaneously, means (not shown) may be applied to U-shaped'supportmember 103 to'move it transversely back and forth relative to sheet 101.In

a this way, the scanning pattern traced out relative to sheet 101- 'isasshown in "broken curve 1G7. Thus, this scan pattern has a component ofmotion parallel to the two-detector line, i;e., a line between apertures1G5 and 106,-;and a cyclical componentat right-angles tothetwofdetec-tor line The length of the cycle is as indicated; on curve107 between points 1G8 and 139. For every complete cycle, sheet-101-moves a translational amount'parallel to the two-detector line, equal tothe distance'Y'between 1693 and 1 39. Under'these circumstances, Ycorresponds to the pitch of the helical scan pattern of the embodimentof FIGURE 1; Vlith the quantities X -and -a referringgto the samedimensions on housing 104-as they did on-housing 17 ofFIGURlE 1, it maybeseen that the "basic relationship of Y Z (X-t-a) holds in thissituation also,

The transverse motion of the detectors and X-raytube 13237011 U-beam 103may bes'upplied byfan electromechanical'drivesystem substantially thesame as system 99 presented in FIGURES. Additionally, however, a

cam may be-introduced into the system so as toreverse cylinder-by saiddetectors; and means for constrainingthe direction of motion of U-beam1G3 each time the detectors and tube reach the left and right hand edgesof test sheet' 101; respectively. I

While'l have shown particular embodiments and rnethod'sofqmy-invention,it will be understood that many modifications may be made withoutdeparting from the spirit thereof, and I contemplate by the appendedclaims to cover an'y suchmodifications as all within the true spirit andscope-of my invention.-

What I claim is:

1'. A flaw detection system comprising: a source of' penetrating rayenergy; means for supporting an object to-be-inspected for flaws; twopenetrating ray detectors, said detectors being disposed'relative tosaid sourceand to said support meanssuch thatfsaid inspection objectwhen mountedion said support means in interposed ,between'said sourceandsaid detectors, whereby each .of said detectors has an effectivefield of view on'and through said inspection object; said detectorsbeing spaced fromteach other alongan imaginary line hereinafter referredto as the tWo-detector 'line; means for providing relative motionbetween said inspection object and said detectors having a translationalcomponent of: motion parallel to said-two-detector lineanda cyclicalcom- ?ponent of motion in a plane perpendicular'to said twodetectorgline; said relative motion providing means in-' cluding means'forconstrainingsaid-translational com;

ponentof motion to a magnitude equal to' or less than V twicethe-sum ofthe distance between the fields of view of said two detectors in thedirectioniofsaid two-detector line and the length-of 'said field of viewin the direction parallelto said two-detector line,- during eachcom-plete cycle of said cyclical'component of motion; whereby no flaw insaid inspection object can at any point in time simultaneously interceptthe fields of'view of both de tectors Without intercepting the field ofview of solely one of said'detectors at-some other point-intime;

2. A'flaw' detection system comprisingz a source of penetrating myenergy; acylindrical test obje'ctdisposed with its longitudinalaxisperpendicular to-the axis of=thc path of penetrating my energy from saidsource; two penetrating ray detectors disposed on opposite sidesof saidcylinder from said source, said'detectors being-spaced from eachotheralong a line parallel to said longitudinal axis of-said cylinderandwithinthe-path of-ray energy passing from-said source out-from saidcylinde'r so that each of said detectors efiectively -has-a field ofviewon and through said cylinder; the bounds of- 'said' fieldsof viewbeing spaced from-each other at their most 'closely adjacent edges bya'distanceXin the direction-parallel to said longitudinal axis, one ofsaid fields of view-having a -dimension' a parallelftot saidlongitudinal axis;- means for moving said cylinder and saiddetectorsrelative to each Otherto-defihe a helioals'canning-cpatternuofusaid said reiative motion tosatisfytherelationship 1 where Y is thepitch of the helix' defined bysaid'helical;

scanning pattern. 7 i

3. A flaw detection system comprising; a: source=of X- rays; means forsupporting a large ,cylindrical testbbject relative to said source ofX-rayslsuch that- X-rays emanating from said source penetrate and'passthmugh the cylindrical test object supportedby said means; two

ionization chambers disposed on the opposite; side of said cylindricaltest object from said vX-ray source; said chambers beirigspaced fromeach'other alonga line paral lel, to the longit'udinaljaxis of'said.:cylindrical test object;

7 X-ray collimating means interposed .in the path of said X-rays betweensaid cylindricaltestobjectand. each 'of said ionizationrchambersiwhereby a parztlleLbeam-of rays enters each, of saidionization chambers and'eachof said ionization chambers therebyeifectively'has a field of View 7 ject by said ionization chambersinth'ata pointaprojected from one ionization chamber in a directionper-pendicular to said longitudinal axis and onto the suiface'of saidcylinder describes a helix having; a pitchY as faresult ofsaidrelativerotational and translationaljmotion; and control, meansincluded in said reiative motion means for constraining said motion tosatisfy the expression:

4. Flaw dctectionapparatus as recited in claim 3 including: circuitmeans coupled to the output orfeach of' said ionization chambers forderiving-a dilfe'rence quantity commensurate with V the differencein'magnitudes. be-

tween said outputsiof said ionization chambers andfref cording meansresponsive to said' circuit means for. re-.

coupled to the output of said thirdlionization chamber and the output ofOne; but not' both ofjsaid two ionization 1 Z chambers for deriving asecond difierence quantity commensurate with the difference inmagnitudes between said third chamber output and said one chamberoutput, the output of said second circuit means being coupled to saidrecording means for recording said second difference quantity on asecond channel of said recording means.

References Cited by the Examiner UNITED STATES PATENTS 2,094,318 9/37Failla 250-4336 2,097,760 11/37 'Failla 31393X 1 8 2,525,292 10/50 Fua250S3.4 2,885,557 5/59 Kizaur 25083.4 2,928,947 3/60 Cherry 250522,983,819 5/61 Bigelow 25083.4 5

FOREIGN PATENTS 567,280 2/45 Great Britain.

RALPH NELSON, Primary Examiner. ARTHUR GAUSS, JAMES W. LAWRENCE,

Examiners.

1. A FLAW DETECTION SYSTEM COMPRISING: A SOURCE OF PENETRATING RAYENERGY; MEANS FOR SUPPORTING AN OBJECT TO BE INSPECTED FOR FLAWS; TWOPENETRATING RAY DETECTORS, SAID DETECTORS BEING DISPOSED RELATIVE TOSAID SOURCE AND TO SAID SUPPORT MEANS SUCH THAT SAID INSPECTION OBJECTWHEN MOUNTED ON SAID SUPPORT MEANS IN INTERPOSED BETWEEN SAID SOURCE ANDSAID DETECTORS, WHEREBY EACH OF SAID DETECTORS HAS AN EFFECTIVE FIELD OFVIEW ON AND THROUGH SAID INSPECTION OBJECT; SAID DETECTORS BEING SPACEDFROM EACH OTHER ALONG AN IMAGINARY LINE HEREINAFTER REFERRED TO AS THETWO-DETECTOR LINE; MEANS FOR PROVIDING RELATIVE MOTION BETWEEN SIDINSPECTION OBJECT AND SAID DETECTORS HAVING A TRANSLATIONAL COMPONENT OFMOTION PARALLEL TO SAID TWO-DELECTOR LINE AND A CYCLICAL COMPONENT OFMOTION IN A PLANE PERPENDICULAR TO SAID TWODETECTOR LINE; SAID RELATIVEMOTION PROVIDING MEANS INCLUDING MEANS FOR CONSTRAINING SAIDTRANSLATIONAL COMPONENT OF MOTION TO A MAGNITUDE EQUAL TO OR LESS THANTWICE THE SUM OF THE DISTANCE BETWEEN THE FIELDS OF VIEW OF SAID TWODETECTORS IN THE DIRECTION OF SAID TWO-DECTECTOR LINE AND THE LENGTH OFSAID FIELD OF VIEW IN THE DIRECTION PARALLEL TO SAID TWO-DECTECTOR LINE,DURING EACH COMPLETE CYCLE OF SAID CYCLICAL COMPONENT OF MOTION; WHEREBYNO FLAW IN SAID INSPECTION OBJECT CAN AT ANY POINT IN THE TIMESIMULTANEOUSLY INTERCEPT THE FIELDS OF VIEW OF BOTH DETECTORS WITHOUTINTERCEPTING THE FIELD OF VIEW OF SOLELY ONE OF SAID DETECTORS AT SOMEOTHER POINT IN TIME.